[英]Inheritance and Try-With-Resources
假设有两个类实现了AutoCloseable
接口,如下所示:
public class Closing1 implements AutoCloseable {
private boolean closed;
@Override
public void close() throws Exception {
if (closed) {
throw new Exception("Closed Already");
}
this.closed = true;
System.out.println("Closing1 closed");
}
public boolean isClosed() {
return closed;
}
}
和
public class Closing2 implements AutoCloseable {
private Closing1 cl1;
public Closing2(Closing1 cl1) {
this.cl1 = cl1;
}
@Override
public void close() throws Exception {
if(!cl1.isClosed()) {
throw new Exception("Closing1 not closed");
}
System.out.println("Closing2 closed");
}
}
我发现尝试资源的所有变化都会导致异常! 我在这里缺少什么,还是只是TWR的设计方式?
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
} //Exception while auto closing C2
要么
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
c1.close();
} // exception while auto closing c1
Try-with-resources将按照声明的相反顺序关闭资源。 这意味着将首先调用c2.close()
,这将在编码后引发异常。
首先从try-with-resources开始, https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
如第一个示例所示:
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
}
人们不一定会命名链中的所有内容。
除非您明确需要c1(关闭以外),否则在现实生活中,您的代码片段看起来像
try(Closing2 c2 = new Closing2(new Closing1())){
System.out.println("Done");
}
并且您肯定不会在try块中调用c1.close()
,因为根本没有c1。
记住这一点,因为所包含的c1没有关闭而从c2引发异常是完全错误的,实际上c2拥有Closing1对象并应在其上调用close()
:
class Close1 implements AutoCloseable {
@Override
public void close() throws Exception {
System.out.println("Closing c1");
}
}
class Close2 implements AutoCloseable {
Close1 c1;
Close2(Close1 c1) {
this.c1=c1;
}
@Override
public void close() throws Exception {
System.out.print("Closing c1 from c2: ");
c1.close();
System.out.println("Closing c2");
}
}
void test() {
System.out.println("Before try block");
try(Close2 c2=new Close2(new Close1())) {
System.out.println("In try block");
}
catch(Exception ex) {
System.out.println("Exception: "+ex);
}
finally {
System.out.println("In finally block");
}
System.out.println("After try block");
}
但是,如果有人给c1命名,它将被关闭两次,这就是幂等性出现在图片中的位置,正如某人已经建议的那样:
System.out.println("Before try block");
try(Close1 c1 = new Close1(); Close2 c2 = new Close2(c1)){
System.out.println("In try block");
}
catch(Exception ex){
System.out.println("Exception: "+ex);
}
finally{
System.out.println("In finally block");
}
System.out.println("After try block");
正如前面提到的BufferedReader
,这是它的close()
方法:
public void close() throws IOException {
synchronized (lock) {
if (in == null)
return;
try {
in.close();
} finally {
in = null;
cb = null;
}
}
}
如果它具有in
,它将被关闭并为空(在finally块中,因此即使发生异常也会发生),并且所有这些都在线程安全的块中。 ( cb
只是一个字符数组,它也会被归零,从而稍微简化了垃圾收集器的寿命)。 由于将finally块中的所有内容都设为空,因此对该同一个方法的任何额外调用都不会做任何事情(除了暂时同步锁之外)。
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